Viral DNA recovered from 4,500-year-old human remains shed new light on the evolutionary past of hepatitis B virus (HBV), and some researchers have suggested that up to two thirds of Bronze Age Mongol warriors may have been living with the virus, ArsTechnica reports.

Medical researchers had until now believed that hepatitis B went back only 450 years, a belief based on a sample found in an earlier study. But after sequencing more than 300 genomes from the skeletons of people who lived on the Asian steppe during the Iron and Bronze Ages, evolutionary scientists at Cambridge University have found much older traces of the virus. According to study authors, 25 out of 304 people were found to have hepatitis B DNA in their bones, suggesting the virus was already widespread thousands of years ago.

The surprising discovery was made only after researchers considered so-called extraneous sequences of DNA not associated with the human genome. Typically, software gets rid of these sequences from remains. But Cambridge researchers dug deeper for this study in order to search for forgotten viral genomes.

The findings challenge a number of theories around the origin of the hepatitis B virus. Some theories have assumed that HBV came from America and made its way to Europe only 500 years ago. Other theories claimed that hepatitis B emerged in Africa and spread to the Americas and India via the slave trade. But the Cambridge researchers posit that the virus emerged in in modern-day southwest Russia 4,300 years ago.

Researchers also said it is possible that they may find even older traces of hepatitis B or other blood-borne viruses in future samples, revealing an even deeper history of the virus.